1. Field of the Invention
The invention relates to the simultaneous extraction of bitumen from tar sands and the recovery of asphaltenic and maltenic material therefrom. More particularly, it concerns the use of two or more solvents of differing polarity to effect such extraction and recovery.
2. Discussion of the Prior Art
Solvent deasphalting is a well-known operation in refineries. The primary objective of deasphalting is to separate a petroleum stream into a relatively high quality fraction ("deasphalted oil" or "maltenes") and a lower quality fraction ("asphalt" or "asphaltenes"). The asphaltenes are generally higher in aromaticity, Conradson Carbon Residue (CCR), molecular weight, and heteroatom (S, N, Ni, V, Fe, etc.) content. Conversely, the maltenes are generally lower in these. Solvent deasphalting may also be thought of as a simple form of compositional delineation. Many methods have been used in the past, and many are currently available for separating hydrocarbons into their more distinct compositional groups. One available method is deasphalting with liquid propane.
Bitumen from tar sands in one example of useful petroleum streams suitable for solvent extraction. Its recovery is conventionally carried out using the Clark hot water process, in which the tar sands material is mixed in a 160.degree. to 180.degree. F. caustic-conditioned water bath. The presence of the caustic, by surface tension lowering, aids in the removal of the bitumen from its sand/clay host. The solids are allowed to settle out and the water and bitumen form two immiscible phases. Following secondary separation the water is sent to a tailings pond, where remanent oil collects on the surface. The bitumen is distilled into major boiling range fractions, and the distillate fractions are catalytically hydrodesulfurized. The residual fraction is conventionally coked.
Other extraction processes for tar sands have been developed which employ solvents or aqueous polymer solutions. All of these, however, require further separation of the low quality asphaltenese from the desired fraction. In addition, the bitumen can be removed by retorting, and although this process has lower water requirements, there is a significant liquid yield loss due to nonselective thermal cracking to coke and gaseous products.